Vishal Digital Electronics Pvt. Ltd., Vitthalpur, Sikhar, Mirzapur

Time to live For the 1969 French film, see  Time to Live (film) . Time to live  ( TTL ) or  hop limit  is a mechanism which limits the lifespan or lifetime of data in a computer or network. TTL may be implemented as a  counter  or  timestamp  attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded or revalidated. In  computer networking , TTL prevents a data packet from circulating indefinitely. In  computing  applications, TTL is commonly used to improve the performance and manage the  caching  of data. IP packets Under the  Internet Protocol , TTL is an 8-bit field. In the  IPv4 header , TTL is the 9th  octet  of 20. In the  IPv6 header , it is the 8th octet of 40. The maximum TTL value is 255, the maximum value of a single octet. A recommended initial value is 64. [1] [2] The time-to-live value can be thought of as an upper bound on the tim...

THE RF CHALLENGES OF ATC COMMUNICATIONS David Hand C.Eng MIET Consultant Engineer - Park Air Systems Ltd. Abstract - The backbone of an Air Traffic Control system is the communication network allowing  controllers to communicate with pilots. Every civilian aircraft should be able to communicate with any  ATC system across the world which leads to unique challenges in establishing and maintaining a  standard system. Against a background of enormous growth in air traffic, upgrading a  communications system on a worldwide basis with many different stakeholders has and will continue  to place considerable demands on the RF parts of the communications networks. This non- mathematical paper will present aspects of the RF performance demanded of ground based ATC  radios and the communications systems they are used in. 1. Radios used in Civil ATC Radios used to communicate between aircraft and the ground have historically never been the same  at each end of th...

A  short dipole  is a simple wire antenna. One end of it is open-circuited and the other end is fed with AC source. This dipole got its name because of its length. Frequency range The range of frequency in which short dipole operates is around 3KHz to 30MHz. This is mostly used in low frequency receivers. Construction & Working of Short Dipole The  Short dipole  is the dipole antenna having the length of its wire shorter than the wavelength. A voltage source is connected at one end while a dipole shape is made, i.e., the lines are terminated at the other end. The circuit diagram of a short dipole with length L is shown. The actual size of the antenna does not matter. The wire that leads to the antenna must be less than one-tenth of the wavelength. That is L &lt; &#x03BB; 10 " role="presentation"> L < λ 10 L < λ 10 Where L  is the length of the wire of the short dipole. λ  is the wavelength. Another type of short dipole is infinitesimal dip...

If the length of the dipole, i.e. the total wire, equals the full wavelength  λ , then it is called as  full wave dipole . If a full wavelength dipole is used either for transmission or for reception, let us see how the radiation will be. Construction & Working of Full-wave Dipole The full-wave dipole with its voltage and current distribution is shown here. Both the positive and negative peaks of the wave induce positive and negative voltages respectively. However, as the induced voltages cancel out each other, there is no question of radiation. The above figure shows the voltage distribution of full-wave dipole whose length is  λ . It is seen that two half-wave dipoles are joined to make a full-wave dipole. The voltage pattern when induces its positive charges and negative charges at the same time, cancel out each other as shown in the figure. The induced charges make no further attempt of radiation since they are cancelled. The output radiation will be zero for a fu...

A folded dipole is an antenna, with two conductors connected on both sides, and folded to form a cylindrical closed shape, to which feed is given at the center. The length of the dipole is half of the wavelength. Hence, it is called as  half wave folded dipole antenna . Frequency range The range of frequency in which half wave folded dipole operates is around 3KHz to 300GHz. This is mostly used in television receivers. Construction & Working of Half-wave Folded Dipole This antenna is commonly used with the array type antennas to increase the feed resistance. The most commonly used one is with Yagi-Uda antenna. The following figure shows a half-wave folded dipole antenna. This antenna uses an extra conducting element (a wire or a rod) when compared with previous dipole antenna. This is continued by placing few conducting elements in parallel, with insulation in-between, in array type of antennas. The following figure explains the working of a half-wave folded dipole antenna, whe...

The dipole antenna is cut and bent for effective radiation. The length of the total wire, which is being used as a dipole, equals half of the wavelength (i.e., l = λ/2). Such an antenna is called as  half-wave dipole antenna . This is the most widely used antenna because of its advantages. It is also known as  Hertz antenna . Frequency range The range of frequency in which half-wave dipole operates is around 3KHz to 300GHz. This is mostly used in radio receivers. Construction & Working of Half-wave Dipole It is a normal dipole antenna, where the frequency of its operation is  half of its wavelength . Hence, it is called as half-wave dipole antenna. The edge of the dipole has maximum voltage. This voltage is alternating (AC) in nature. At the positive peak of the voltage, the electrons tend to move in one direction and at the negative peak, the electrons move in the other direction. This can be explained by the figures given below. The figures given above show the work...

Wire antennas are the basic types of antennas. These are well known and widely used antennas. To have a better idea of these wire antennas, first let us have a look at the transmission lines. Transmission Lines The wire or the  transmission line  has some power, which travels from one end to the other end. If both the ends of transmission line are connected to circuits, then the information will be transmitted or received using this wire between these two circuits. If one end of this wire is not connected, then the power in it tries to escape. This leads to wireless communication. If one end of the wire is bent, then the energy tries to escape from the transmission line, more effectively than before. This purposeful escape is known as  Radiation . For the radiation to take place effectively, the impedance of the open end of the transmission line should match with the impedance of the free-space. Consider a transmission line of a quarter-wave length size. The far end of it...